Not sure, I like hearing my bike maybe. Never sure to be honest, quiet bikes are very foreign to me, so when I "hear" them I usually do a double take to figure out what hubs they are running. I wouldn't say it is limited to downhill/freeride bikes but I think those are the primary users. My hadley for example, while loud, has by far outlasted any shimano hub I have.

As for why, I am not an expert, but I would it might have something to do with number of engagement points but more to do with design. Types or materials etc. I relate it to engagement points. If I hear a lot of noise it usually a hadley, king or hope.